1. Field of the Invention
Example embodiments of the present invention relate to a memory device, and more particularly, to a memory device with an external magnetic field generator and method of operating and manufacturing the same.
2. Description of the Related Art
Magnetic memory devices, together with ferroelectric random access memory (FRAM) devices, parameter random access memory (PRAM) devices, and redundant random access memory (RRAM) devices, are considered to be the next generation of non-volatile memory devices. A magnetic memory cell may include a field effect transistor (FET) as a switching unit and a magnetic tunnelling junction (MTJ) cell. Such a magnetic memory device is characterized by the MTJ cell. The MTJ cell may include a lower magnetic layer of which magnetization orientation is fixed, an upper magnetic layer (free magnetic layer) of which magnetization orientation may be changed by an external magnetic field, and a tunnelling layer interposed between the lower and the upper magnetic layers.
In a conventional MTJ cell of a magnetic memory device, the magnetic orientation of the free magnetic layer is determined by a vector combination of a magnetic field generated by a bit line in a first (for example, an easy) axis direction and a magnetic field generated by a digit line in a second (for example, a hard) axis direction, where the bit line and the digit line are perpendicular to each other. Because the free magnetic layer has a residual magnetization, the free layer may maintain its magnetization orientation even when the magnetic memory device is powered-off after the magnetic orientation of the free magnetic layer is determined. That is, because the magnetization orientation of the free magnetic layer means stored data, the data in the MTJ cell may be retained even when not powered.
Magnetic memory devices have advantages, for example, a good non-volatile characteristic and a simple structure suitable for memory integration. However, the MTJ cell of the magnetic memory device should have a large magnetic resistance ratio (MR ratio) to secure a sufficient sensing margin. Also, the magnetic coupling between the lower magnetic layer and the free magnetic layer should be reduced. Specifically, cell selectivity may be reduced as the integration level of the memory device increases. That is, because the distances between the MTJ cells of the magnetic memory device become smaller as the integration level increases, non-selected MTJ cells around a selected MTJ cell may be affected by the magnetic field of the bit line. Therefore, the data in the non-selected MTJ cells may be overwritten, deleted, or destroyed.
To obviate this problem, a magnetic memory device utilizing a local field has been introduced. This type of magnetic memory device is capable of concentrating the local field on a selected MTJ cell to prevent non-selected MTJ cells from being affected by the local magnetic field, thereby increasing the cell selectivity.
The local field type magnetic memory device, however, has a drawback in that the coercivity of the MTJ cell is increased when the distance between the MTJ cells decreases as the integration level of the magnetic memory increases. That is, a larger current is required to write data on or delete data from a selected MTJ cell. Further, the size of a transistor reduces as the integration level of the magnetic memory device increases and thereby the current limit of the transistor also reduces. Therefore, the write current of the MTJ cell is limited to the current limit of the transistor.
Further, when the MTJ cell is larger than 0.4 μm (when the integration level of the local field type magnetic memory device is lower than 4 MB), the effect of the local field is reduced. Therefore, a large current is also required in this case.
As described above, because the local field magnetic memory device requires a large current in both cases, e.g. when its integration level is high and low, an additional digit line may be required to lower the current.
However, many additional processes may be required to add the digit line to the magnetic memory device, thereby decreasing manufacturing productivity thereof. Also, the power consumption of the magnetic memory device increases because a current must be applied to both the bit line and digit line.